1. Field of the Invention
The present invention relates to a means and method for imaging an optical path in a laser oscillator/amplifier and, more particularly, for imaging an optical path in an amplifier-phase conjugate mirror (PCM).
2. Description of Related Art and Other Considerations
In the prior art, a vacuum is used in an imaging telescope to eliminate the problem of air breakdown, which occurs when laser energy is focused at a single point, for example, discussed in a report AL 88-22 dated Nov. 4, 1988 and entitled "Medium Average Power Solid-State Laser Technical Information Seminar" for the Lawrence Livermore National Laboratory of the University of California, and as described with respect to FIGS. 1-4.
FIGS. 1 and 2 schematically show how a phase conjugate mirror 10, having a window 11, is used to compensate aberrations induced by a solid state laser amplifier 12. As illustrated in FIG. 2(a), an input beam 14, whose wave is identified by indicium 16, is produced by an oscillator 13 (shown in FIGS. 7 and 8) with a spatially uniform phase front which is distorted or deviated, as represented by wave 18 as it passes through some medium 20 in the amplifier in some arbitrary way, generally by thermal conditions affecting the crystal of amplifier 12. An ordinary mirror inverts the distortion as it reflects the beam, shown by wave 22, thereby keeping the distortion fixed with respect to the propagation direction and, upon a second pass through amplifier medium 20, the distortion is doubled, shown by wave 24. As depicted in FIG. 2(c), however, phase conjugate mirror 10 creates a backward going wave 26, known as a Stokes wave, through a process known as stimulated Brillouin scattering. Because the Stokes wave is a phase reversal of the input wave, the same region 20 of the amplifier, which originally created the distortion, now compensates for this distortion as the beam makes a second pass through the amplifier, to exit with the same front shown as wave 28 as the original front of wave 16.
The degree, to which the Stokes wave is an exact conjugate of the pump wave, is determined by the fidelity of the phase conjugation. An important condition for this high fidelity conjugation to occur is that all of the pump wave must be collected by the aperture of the phase conjugate mirror; otherwise, information about the input wave will be lost. Because of effects arising from diffraction or thermal distortion, complete collection of the pump wave is often difficult to achieve where outer portions 14a of beam 14 diverge and may be truncated, such as is shown in FIG. 1.
Anything less than full collection reduces the fidelity of the phase conjugation, in that a portion of the laser energy is not collected at the PCM aperture and, therefore, does not become a factor in the development of the Stokes field. A common method of correcting this problem is to use an imaging system in which, as shown in FIG. 3, the beam must be relayed by positive lenses 30 and 32. Lenses 30 and 32 are used to form an imaging telescope, as this preserves the collimation of the beam. The lenses are separated the distance f.sub.1 +f.sub.2, which makes the system afocal. The object and image distances S.sub.1 and S.sub.2 from the nearest lens are given by the equation EQU F.sub.1 +f.sub.2 =mS.sub.1 +S.sub.2 /m,
where m=f.sub.2 /f.sub.1.
A problem with imaging in general is that an intermediate focal point 34 (see also FIG. 4) is created. In medium and high power Q-switched laser systems, air breakdown will occur at this focus, causing a plasma arc that completely absorbs the laser radiation. As stated above, this air breakdown problem is normally overcome by placing a vacuum at the focus. Because of optical damage limitations, the use of a vacuum often requires that the entire telescope be integrated with a vacuum chamber, and this integration adds considerably to the complexity and cost of the telescope.
Therefore, the method using a vacuum to eliminate the air breakdown problem, while effective, is a complex and costly method which adds cost and weight to the system. In addition, the integrity of the vacuum is dependent, inter alia, upon the vacuum seal which, for long-lifetime applications, must be a hermetic seal.